In the era of the 1980's, state-of-the-art aircrew ensembles, such as, helmet-mounted displays, night vision systems, and chemical/biological defense gear, were introduced. The increased envelope of these ensembles renewed concern about the need for the flip-up pitots system. Investigations focusing on the increased size of the helmet envelope, and its aerodynamic interference with fixed pitots airspeed inlets, proved that improper function of the Ejection Seat Sequencer could occur (John B. Arnaiz, Gentex Corporation, "Helmet Assembly Aerodynamic Interference Effects on ACES II Ejection Seat Operation," SAFE PROCEEDINGS 1985). These investigations, coupled with aircrew complaints of helmet interference and vision difficulties with fixed pitots system, increased interest in upgrading ejection seats, particularly the ACES II Ejection Seat, with a flip-up pitots system.
ACES II Ejection Seat escape performance depends entirely on the Pitots System, the "airspeed sensor" of the ejection seat, which provides critical airspeed environmental data to the seat's electronic recovery sequencer upon ejection.
As depicted in FIG. 1, ACES II pitots systems utilize two (2) pitots tubes 10 mounted on the left and right hand sides of the recovery parachute container assembly 12, located behind the headrest. They are oriented such that the open ends (airspeed inlets) face into the air stream. Pitots are designed to determine the seat's actual airspeed upon ejection, allowing correct mode selection to occur. As the ACES II Ejection Seat travels up the guide rails during an ejection and as the seat exits the cockpit, the pitots become exposed to the air stream and collect critical airspeed pressure data. The Pitots System of the ACES II Ejection Seat is used to control critical ejection seat and recovery system event timing, based on its airspeed input data, which is sent to an environmental sensor and then to a recovery sequencer. The recovery sequencer then selects one of three specific ejection modes dependent on altitude and speed at the start of the ejection sequence. It is the pitots system input data which determines one of three ejection modes for the seat. These inputs allow a recovery sequencer to select an ejection mode that provides for safe recovery of the aircrew member in the actual escape conditions.
Fixed pitots systems are mounted to the recovery parachute container assembly in a stationary position so that they are always deployed, even when they are not operational, as depicted in FIG. 1. Because Fixed Pitots are always deployed, their configuration is limited and governed by clearances with the surrounding cockpit structure. The two fixed pitots 14 typically have only a 16-inch (40 cm) span between the extended sensing ports of the right and left pitots. In addition, each of the pitots is oriented at an angle of 65 degrees up from vertical, locating the sensing port in the proximity of turbulent airflow around the Aircrew member's helmet. This arrangement compromises the critical pitots airspeed input data to the Recovery sequencer, which places the ejecting aircrew in risk of injury or death from incorrect ejection mode selection due to incorrect critical pitots airspeed input data.
In contrast, as shown in FIG. 2, flip-up pitots systems are mounted near the top of the recovery parachute container assembly. While FIG. 2 depicts the flip-up pitots in their deployment position, typically they remain in a stowed configuration until an ejection occurs and they are deployed. This stowed configuration provides many advantages: the stowed pitots remain out of the rear vision area of aircrew members, it poses minimal risk of helmet impact and it is also protected from inadvertent damage during servicing operations. During an ejection, the flip-up pitots deploy into the free air stream as the seat travels up the rails. The deployed flip-up pitots configuration has a greater outboard span of 22 inches (55 cm) between the extended airspeed inlet ports of the right and left pitots and an angle of 100 degrees upward position from vertical, locating the sensing ports in a much cleaner air stream away from the aircrew member's helmet. During seat ejection, as the seat travels up the rails, flip-up pitots deployment starts when a spring-loaded bellcrank mounted on the Recovery parachute container assembly moves up past the end of the guide rails. This allows the bellcrank to rotate which pulls a locking pin from the stowed Pitots. A torsion spring then rotates the pitots up until it becomes fully deployed and locked in that position.
In view of the foregoing, the United States Airforce (USAF) determined that fixed pitot systems are unacceptable for general use. In addition, flight personnel and aircrew and maintenance personnel complained that Fixed Pitots interfere with aircrew ability to "check six" (over the shoulder rearward vision) during air combat maneuvers.
The flip-up pitots system was designed as a complete recovery parachute container assembly replacement. Initial production of flip-up pitots systems were installed on B-2 Aircraft. Follow-on flip-up pitots system production was then installed on in-production F-16 Aircraft starting with Block 40 and up. Fixed Pitots Systems in current operational aircraft were then identified by USAF to be replaced with flip-up pitots systems on an attrition basis. As a result of such gradual replacement, users are currently forced to replace the entire Recovery parachute container assembly in order to achieve compliance, a difficult and expensive proposition, especially in the current fiscal environment. The cost of this replacement flip-up pitots recovery parachute container assembly is approximately $4,000.00 per unit.
Thus, it is most desirable to exchange the Fixed Pitots with the flip-up pitots without replacing the recovery parachute container assembly.